1. Field of the Invention
The present invention relates to a knocking detector of an internal combustion engine, and more particularly to an improvement in a knocking detector of an internal combustion engine suitable for use with an ignition timing control device for retarding ignition timing based on detected knocking.
2. Description of the Prior Art
Since engine knock is a destructive phenomenon which may destroy the engine in the worst case, ignition timing has been retarded by a certain predetermined degree from an ignition timing which causes knocking. However, since it is undesirable with respect to fuel consumption to retard the ignition timing from the critical knocking ignition timing, it has been proposed to control ignition timing to the critical knocking ignition timing (that is, the most advanced angle which does not cause knocking) employing feedback. (See Japanese Published Unexamined Patent Application 52-87537).
In the proposed ignition timing control device, the knocking of the engine is monitored and the ignition timing is feedback controlled such that if the knocking is detected the ignition timing is retarded and if the knocking is not detected the ignition timing is advanced to the critical knocking ignition timing. Referring to FIG. 1, the knocking detection device comprises a knocking detection circuit 11 including a knocking sensor 10 which may be an acceleration pick-up or a microphone for detecting vibration in the engine body or sound waves generated by the vibrations and transducing it to an electrical signal, a high-pass filter 12 for blocking the low frequency components in the output of the knocking sensor 10, a signal offset circuit 14 for offsetting the output of the high-pass filter 12 by a predetermined level, a masking circuit 16 for masking a portion of the output of the signal offset circuit 14 to prevent malfunction of the knocking detection device due to an external noise, a half-wave rectifying circuit 18 for half-wave rectifying the output of the high-pass filter 12, an integration circuit 20 and an amplifier circuit 22 for producing a reference level signal corresponding to an engine dark noise level from the output of the half-wave rectifying circuit 18, and a comparator circuit 24 for comparing the output of the masking circuit 16 with the reference level output of the amplifier circuit 22 to determine the occurrence of knocking when the output of the masking circuit 16 exceeds the reference level, and an ignition timing control circuit 26 which receives the output of the comparator circuit 24 as a knocking signal.
In such a knocking detector, the knocking signal is produced when the output of the knocking sensor 10 exceeds the reference level corresponding the engine dark noise level derived by integrating and amplifying the output of the knocking sensor 10. As compared with the case where the reference level is fixed independently of engine dark noise level, the knocking is more exactly determined while the engine is rotating both at low speeds and at high speeds. However, the reference level which directly corresponds to the engine dark noise level increases more rapidly at high engine rotation speeds, as shown by a broken line A in FIG. 2, while the output of the knocking sensor caused by knocking vibrations does not increase significantly at high engine rotation speeds. As a result, the reference level increases much more than the knocking sensor output does during high loads and high rotational speeds. Thus, if the integration time constant and amplification factor of the knocking detection circuit are improperly set, the reference level will be larger than the output of the knocking sensor 10 produced by knocking vibrations so that knocking cannot be detected at all. In such a case, the ignition timing becomes too advanced at high engine rotational speeds as shown by a broken line B in FIG. 3 so that it is more advanced than the ignition timing that would be determined by a governor on the distributor shown by a solid line C in FIG. 3. This may result in engine damage.
In order to resolve the above difficulties, it has been proposed to set the amplification factor for the amplifier low in order to establish a relatively low reference level. In such a case, however, knocking is detected erroneously so that the amount of retardation is too large, causing high fuel consumption and an exhaust gas temperature rise.
It has also been proposed to lower the reference level upon the detection knocking so that the ability to detect the knocking is enhanced when knocking occurs. However, this poses a problem of complexity in the construction of the knocking detection circuit.